Is the following refinance possible?

Secured against my current PPOR (worth approx $475,000), had been payed down to zero, and then redrawn for investment purposes, Loan Balance $350,000 with $200,000 in an offset account (so paying interest currently on $150,000)

and wanted to buy a new PPOR worth approx $900,000 (selling old PPOR). Is it possible to refinance this so that the new loan is secured on the new PPOR, and split like:

Your post is a little confusing.
Are you saying that you want to sell the old PPOR and buy a new 1 worth twice a much but you want to keep the old PPOR investment loan of 350K?
What did you buy with the $150K you have invested?

Paid down the loan balance down to zero, then redraw to buy $350,000 worth of managed funds. Then built up $200,000 in the offset account. The bit I am worried about is when refinancing it for the new loan.

I believe it's doable if you have the correct loan type.
I am thinking of a loan package such as the advantage saver of STGeorge where you can have multiple loans as a package and an offset.
There are similar loan types around with other lenders.

You could use the $200K as a deposit for the new PPOR and
add the new property to your existing loan package.

Your account will now have
LOAN 1 for the old PPOR
LOAN2 for the managed funds
LOAN3 for the new PPOR
OFFSET linked to OLD PPOR and as soon as you move into PPOR 2
you link it to the PPOR 2 loan instead

You could have serviceability issues when you buy PPOR2
but you can tell your lender that the new PPOR will be an investment and you will be getting $XXX/week in rent.
This will help with your serviceability and will increase your ability to borrow the $700K.
Meanwhile you put your old PPOR on the market and sell it.
When it sells, you then deposit the OLD PPOR money into the offset
and have still retained the original IP loan.

The other advantage of having this structure, is that because of the large loan size you could probably get 0.9% discount off their standard variable loan. Even when you sell PPOR1 you will still have a large loan so you should still be able to retain this discount.